`pkg.bzl`: Add features to support the nolang dist.

In order to build the language-independent parts of our dist with `pkg.bzl`,
we need two override features:
* A way to set the prefix in the zip files to the empty string, so that our top-level files
  stay top-level.
* A way to put `codeql.exe` into the arch-specific zip, despite it not being under `CODEQL_PLATFORM`.
This PR implements both.

misc/bazel/pkg.bzl

@@ -96,6 +96,7 @@ def codeql_pkg_files(
 
 def _extract_pkg_filegroup_impl(ctx):
     src = ctx.attr.src[PackageFilegroupInfo]
+    arch_overrides = ctx.attr.arch_overrides
     platform = _detect_platform(ctx)
 
     if src.pkg_dirs or src.pkg_symlinks:
@@ -106,8 +107,11 @@ def _extract_pkg_filegroup_impl(ctx):
         dest_src_map = {}
         for dest, file in pfi.dest_src_map.items():
             file_kind, expanded_dest = _expand_path(dest, platform)
+            if file_kind == "generic" and dest in arch_overrides:
+                file_kind = "arch"
             if file_kind == ctx.attr.kind:
                 dest_src_map[expanded_dest] = file
+
         if dest_src_map:
             pkg_files.append((PackageFilesInfo(dest_src_map = dest_src_map, attributes = pfi.attributes), origin))
 
@@ -125,12 +129,14 @@ _extract_pkg_filegroup = rule(
         destination paths to the relevant codeql platform (linux64, win64 or osx64).
         The distinction between generic and arch contents is given on a per-file basis depending on the install path
         containing {CODEQL_PLATFORM}, which will typically have been added by a `prefix` attribute to a `pkg_*` rule.
+        Files that are arch-specific, but outside of the `CODEQL_PLATFORM` path can be specified in `arch_overrides`.
         No `pkg_dirs` or `pkg_symlink` must have been used for assembling the source mapping information: we could
         easily add support for that, but we don't require it for now.
     """,
     attrs = {
         "src": attr.label(providers = [PackageFilegroupInfo, DefaultInfo]),
         "kind": attr.string(doc = "What part to extract", values = ["generic", "arch"]),
+        "arch_overrides": attr.string_list(doc = "A list of files that should be included in the arch package regardless of the path"),
     } | _PLAT_DETECTION_ATTRS,
 )
 
@@ -277,6 +283,8 @@ def codeql_pack(
         visibility = None,
         install_dest = "extractor-pack",
         compression_level = None,
+        arch_overrides = None,
+        prefix_override = None,
         **kwargs):
     """
     Define a codeql pack. This macro accepts `pkg_files`, `pkg_filegroup` or their `codeql_*` counterparts as `srcs`.
@@ -288,10 +296,13 @@ def codeql_pack(
     * defines a runnable `<name>-installer` target that will install the pack in `install_dest`, relative to where the
       rule is used. The install destination can be overridden appending `-- --destdir=...` to the `bazel run`
       invocation. This installation _does not_ prefix the contents with `name`.
+    The prefix for the zip files can be overriden with `prefix_override`.
 
     The distinction between arch-specific and generic contents is made based on whether the paths (including possible
     prefixes added by rules) contain the special `{CODEQL_PLATFORM}` placeholder, which in case it is present will also
     be replaced by the appropriate platform (`linux64`, `win64` or `osx64`).
+    Specific file paths can be placed in the arch-specific package by adding them to `arch_overrides`, even if their
+    path doesn't contain the `CODEQL_PLATFORM` placeholder.
 
     `compression_level` can be used to tweak the compression level used when creating archives. Consider that this
     does not affect the contents of `zips`, only `srcs`.
@@ -299,6 +310,9 @@ def codeql_pack(
     internal = _make_internal(name)
     zip_filename = zip_filename or name
     zips = zips or {}
+    prefix = name
+    if prefix_override != None:
+        prefix = prefix_override
     pkg_filegroup(
         name = internal("all"),
         srcs = srcs,
@@ -316,6 +330,7 @@ def codeql_pack(
             name = internal(kind),
             src = internal("all"),
             kind = kind,
+            arch_overrides = arch_overrides,
             visibility = ["//visibility:private"],
         )
         if zips:
@@ -335,7 +350,7 @@ def codeql_pack(
                 name = internal(kind, "zip"),
                 srcs = [internal(kind, "zip-base"), internal(kind, "zip-info")],
                 out = _get_zip_filename(name, kind),
-                prefix = name,
+                prefix = prefix,
                 visibility = visibility,
             )
         else:
@@ -343,7 +358,7 @@ def codeql_pack(
                 name = internal(kind, "zip"),
                 srcs = [internal(kind)],
                 visibility = visibility,
-                package_dir = name,
+                package_dir = prefix,
                 package_file_name = _get_zip_filename(name, kind),
                 compression_level = compression_level,
             )